The field of the invention relates generally to electric machines and more specifically to the reduction of leakage current present in electric machines.
Leakage current is produced during the operation of various types of electric machines, for example, electric motors and electric generators. Typically, an electric machine includes a stator and a rotor. The stator includes a plurality of teeth that are wrapped with wire windings. The electric machine may include an insulation between the windings and the stator teeth. This configuration of windings, insulation, and stator teeth creates a capacitance, referred to as machine parasitic capacitance. High frequency switching of voltages applied to the windings creates a potential difference across the parasitic capacitance, and as a result, a circulating current generally referred to as leakage current when the stator enclosure is tied to earth ground. The leakage current across the parasitic capacitance prevents the power lines feeding the system to be balanced, as part of the supplied current returns through earth ground. Electric machines that include internal shielding to reduce the presence of shaft voltages and bearing currents typically couple the shielding to earth ground. However, coupling a shield of this type to earth ground does not reduce leakage current.
Leakage current may cause tripping of a circuit breaker, for example, a circuit breaker included in a ground fault circuit interrupter (GFCI). A GFCI is positioned between a power source and a device to prevent injury due to electric shock. For example, in the United States of America, a typical 120 Volt electrical outlet includes connections for a “hot” wire, a “neutral.” wire, and an “earth ground” wire. The GFCI senses when there is a difference between the electric currents in the hot wire and the neutral wire. Detection of a difference between those currents may indicate a malfunction. For example, a difference between the currents may indicate that electric current is traveling to ground through an unintended path, such as if a person has come in contact with the hot wire and is providing an electrical path to ground. The difference between the currents in the hot wire and the neutral wire may often be qualified as leakage current. Leakage current does not return via the neutral wire, but rather may be present in a ground wire. Tripping of a GFCI due to leakage current may be referred to as nuisance tripping. A reduction of leakage current may prevent nuisance tripping of a GFCI.
The Applicant have disclosed in U.S. Pat. No. 7,952,251 B2 electric machines that include internal shielding to reduce the presence of shaft voltages and bearing currents that couple the shielding to a reference point that is not earth ground. Coupling a shield of this type to a reference point that is not earth ground does reduce leakage current.
The shielding disclosed in U.S. Pat. No. 7,952,251 B2 consists of a unitary sheet that is fed around the stator teeth of the motor and positioned between the teeth and the stator coils. Construction of this shielding device requires the use of very expensive tooling and the installation of this shielding device is very time consuming.
The present invention is directed toward alleviating at least some of the above mentioned difficulties with the prior art.